One of the advantages of horizontal drilling is the increase in productivity of each well. The aim of this article is to propose bases for making an analysis, such as areal productivity, so as to help the decision-maker responsible for the development of a field when confronted with an economic choice between a conventional system using vertical wells and an alternative system including other types of wells, and in particular horizontal wells. Simple and approximate solutions are proposed for evaluating the number of vertical wells that can be replaced by a horizontal wells.